This invention is related to wireless networks, and in particular, to a method and apparatus for the early detection of false receiver start-of-packet (SOP) triggers at a station (STA) in a packet-based wireless network such as a wireless local area network (WLAN).
Wireless networks are known and their use is becoming widespread. The IEEE 802.11 standard for local area networks describes variants in the 2.4 and 5 GHz range. A node of a wireless network such as one conforming to one of the variants of the IEEE 802.11 standard includes a radio transceiver/modem providing physical layer level (PHY) processing and a MAC processor. The receiver of the physical layer processor (the PHY processor) needs to accurately detect the SOP. Conventional receivers use a received signal strength indicator (RSSI) for so detecting the SOP. See for example, U.S. patent application Ser. No. 10/095,668, titled AUTOMATIC GAIN CONTROL AND LOW-POWER START-OF-PACKET DETECTION FOR A WIRELESS LAN RECEIVER to inventors Ryan, et al, filed: Mar. 8, 2002, Attorney/Agent Ref. No. CISCO-5227, that describes a low-power SOP detector based on RSSI. Patent application Ser. No. 10/095,668 is incorporated herein by reference. In addition, the IEEE 802.11a and 802.11g (OFDM) standards include sending a SIGNAL field in the first section of the packet that describes the modulation type and coding rate for the payload of the packet. The SIGNAL field is not very well protected—only a single parity bit is provided. As a result, there are many false triggers where the parity bit falsely indicates a valid SIGNAL field. As a result, the wireless node of the network is unnecessarily blocked from receiving valid packets while it processes what is effectively useless information. Moreover, the wireless node will not perceive that this is a bad packet until it has completely finished processing the packet and detected that the payload fails its frame checksum (FCS) check, which is much more robust than the single parity bit protecting the SIGNAL field.
Thus there is need in the art for fast and early detection of false SOP triggers.